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"Alternative to Zn-Fe plating + Trivalent Black Passivization with less cost?"

February 17, 2016

Q. Is there any Alternate coating for Zn-Fe plating + Trivalent Black Passivization (like ED coating, etc.,) with less cost? Our customer requirement is SST to withstand 120 hours of white rust & 480 hours of red rust. My supplier is not capable of Zn-Fe plating + Trivalent Black Passivization, and they suggest only for Zinc plating + Trivalent Black Passivization. Please suggest if there is any other alternate process (like ED coating, Powder coating, etc., if any)

Lokesh L
Automotive - Bangalore

February 2016

A. Hi Lokesh. To know whether an organic coating like ED (electrodeposition) or powder coating can substitute for a cathodic protection coating like zinc-iron plating we would need to know the application (the type of parts, the environment, etc.) because the protection mechanisms are fundamentally different. However, if you employ the reductionism of using salt spray hours to determine fitness, this does give organic coatings a big unjustified boost up -- because zinc and zinc alloy platings are designed to rapidly dissolve in the artificial environment of salt spray testing whereas organic coating materials are inert to it.

For small parts, plain zinc plating is probably the least expensive surface finish. Going to zinc-iron bumps up the cost a bit (not too much), but it's still a very economical coating, perhaps second only to plain zinc plating in low cost. For larger parts, organic coatings can probably compete price-wise.

But I think the heart of your situation is that you actually want and need zinc-iron plating with trivalent black chromate, and your vendor doesn't want to offer it. Plain zinc plating is not an adequate substitute. I would urge you to find a vendor who will do the plating you want (these days it's not always a one-way street; it may require a long term agreement for your business).

That's my personal take; hopefully other readers will offer theirs. Good luck.


Ted Mooney, finishing.com
Ted Mooney, P.E.
Striving to live Aloha
finishing.com - Pine Beach, New Jersey

February 19, 2016

When you say your customer requirement is SST, I assume that is a typo or you meant something other than what people normally use SST for, which is stainless steel.

If you are using stainless steel, you're asking about the wrong type of passivation treatment entirely, and the white rust requirement you mentioned would be irrelevant.

Otherwise, pass along those performance requirements to your vendor and ask if they have anything that can meet that.

ray kremer
Ray Kremer
Stellar Solutions, Inc.
supporting advertiser
McHenry, Illinois
stellar solutions banner

Ed. note: By "SST", Lokesh apparently means "salt spray testing" rather than "stainless steel".

March 24, 2016

A. Dear lokesh,
Please try some thick film black chromate with TOP COATS having Cr3 & Zr which helps to meet 120 Hours to white rust.

plating professional - chennai, TN, India

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